Particle counters of the stated type are commonly used to determine the biological and hematological cell counts as well as various industrial particle counting uses. Such counters rely on electronically-generated pulses generated by the passage of a particle through the orifice, which pulses are counted electronically and displayed or otherwise read by the test apparatus. As the density of the particles in the fluid carrier increases, the probability of particles being coincident during the detection in the orifice increases, resulting in a reduced count when compared to the actual number of particles passing through the orifice. For a given orifice, and for a given character of particle, the number of such coincident particles may be statistically determined for a specific density of particles per unit volume of solution and conversion parameters may be provided for translating the electronic count to an actual count, without the necessity for further diluting the sample so as to reduce the density of the particles in the sample and thereby the likelihood of particles passing through in coincidence.
Prior to the present invention, coincidence correction has been accomplished by providing correction tables for the readings generated by the electronic count which the operator may use to convert the electronic count to the actual count. Such a procedure is time consuming and introduces a risk of human error. A proposal for electronically storing the correction tables and correcting the digital output by the stored data, as described in U.S. Pat. No. 3,864,551, was found to be uneconomical. Another means of providing coincidence correction is to simply provide an analog read-out and provide a correction in the scale of the analog read-out meter. However, the inaccuracies inherent in an analog read-out are undesirable.
An electronic correction is disclosed in U.S. Pat. No. 3,626,164 wherein electronic circuitry is provided to periodically feed correction signals to the counter in accordance with a schedule determined by the statistical probability of coincidence related to the cumulative total in the counter, so that a corrected count may be read from the digital counter. The intermittent correction of this patented counter requires the addition of a multitude of pulses at spaced intervals of correction which affects the accuracy of the corrected count variably according to the interval between the corrections.
With the foregoing in mind, the present invention makes use of the well-known principles and readily available components to provide a single-increment correction at periodic intervals, the length of the intervals between corrections being modified as the statistical data requires a different degree of correction.
In a preferred embodiment of the invention, the correction is accomplished by the use of two counters, one to record the count of the detected particles with the coincidence correction pulses added to it so that it may be used with a digital display to show the true particle count or with other read-out devices. A second counter is used to receive the actual detected count and to control the addition of correction pulses to the detected count fed to the first counter as the count progresses. The second counter therefore provides a correction pulse at the predetermined intervals by using the same counter, both as a pulse generating means and as a timing means.